1. Field of the Invention
This invention relates to an optically coupled semiconductor device known as an optocoupler. More specifically, this invention relates to the temperature compensation of the current transfer ratio of an optocoupler.
2. Description of the Related Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or form part of common general knowledge in the field.
Optocoupler is a common semiconductor device uses in the electronic system level design for input to output safety or signal isolation. FIG. 1 shows the circuit description and pin out of an optocoupler. The input of an optocoupler consists of an infrared light emitting diode (LED) die. The output of an optocoupler is consists of a detector die which typically is a photo bipolar NPN transistor. The input and output dice are separated by an electrically insulated material which is transparent to infrared light. The input and output components and their light transmitting medium are encased in molded plastic, metal can or ceramic enclosure. When there is current flowing in the LED, it emits infrared light. The infrared light goes through the insulation material and is detected by the collector base region of the photo bipolar NPN transistor. The infrared light generates hole electron pairs in the collector base region of the transistor which resulting a photo current flowing into the base of the bipolar transistor. This photo current is multiplied by the current gain, beta, of the bipolar transistor to produce an output collector current. A current transfer ratio, CTR, of an optocoupler is defined as the ratio of the output collector current over the current flowing in the infrared LED. FIG. 2 illustrates the CTR of an optocoupler. The CTR value of an optocoupler is a key parameter for system level designer especially when the optocoupler is used in the linear operation mode as in the feedback path of a switching mode power supply design as an example. Unfortunately, the CTR of the prior art optocoupler has a very large negative temperature coefficient. The CTR temperature dependency comes from two sources. The first source is the decreases in light emission efficiency of the infrared LED. Typically, the light emission efficiency of an infrared LED has a negative temperature coefficient of about −7500 ppm. So, the LED light emission efficiency decreases to about 40-50% of the room temperature value at 100 C which means that the photo current produces at the base of the output photo transistor is reduced by the same amount. The second source is the change in the beta of the output photo transistor due to increase in emitter efficiency. The beta has a temperature coefficient of about +6000 ppm so the beta value will increase by 40% from room temperature to 100 C. Since the photo current is reduced by 50-60% but the beta only increased by 40%, the output collector current is 60-70% of the room temperature value. Thus the CTR of the optocoupler at 100 C is typically about 60-70% of the room temperature value. This large change in CTR over temperature presents a design difficulty to system level design engineer. To compensate for the large CTR temperature change of the optocoupler, system designer has to add additional components and circuit complexities to the design which increases the system product cost. A mean of such compensation is illustrated in U.S. Pat. No. 7,326,947 where the CTR of the optocoupler system is compensated by introducing an additional temperature compensating current to the LED. However, a way to compensate for the CTR temperature variation while keeping the optocoupler pin out configuration and without additional external input or output design requirement to the optocoupler would be valuable to those who use the optocoupler in their system level design and application.